This invention relates to rubber compositions capable of forming golf balls having an appropriate hardness and improved rebound characteristics and exhibiting stabilized flight performance, and golf balls using the same.
In general, one-piece golf balls or the solid cores of multilayer structure solid golf balls having a solid core enclosed with a cover directly or via an intermediate layer are manufactured by molding under heat and pressure a rubber composition comprising a rubber component such as polybutadiene rubber, a metal salt of an unsaturated carboxylic acid as a co-crosslinking agent, zinc oxide as a weight modifier, and a free-radical initiator such as dicumyl peroxide.
With respect to the metal salt of an unsaturated carboxylic acid used as the co-crosslinking agent in the above rubber composition, use of zinc methacrylate and zinc acrylate is believed preferable from the standpoints of ball hardness and rebound. Typically zinc methacrylate and zinc acrylate are blended in large amounts with the rubber component such as polybutadiene. When a rubber composition is obtained by mixing or kneading the respective components, the kneading operation is very difficult because the zinc salt in fine powder form scatters much and heavily sticks to the roll mill or kneader.
Since the zinc unsaturated carboxylates, when blended in the compositions, are poorly dispersible and liable to form agglomerates in the compositions, they are not effectively utilized, sometimes inviting a loss of resilience and a failure to acquire a certain hardness.
One common approach for avoiding the unsaturated carboxylic acid metal salts from sticking to the kneader is to use metal salts of higher fatty acid as a lubricant. In order for these fatty acid metal salts to exert the desired effects, the kneading temperature must be elevated to as high as about 100° C., but a problem of scorching occurs at such elevated temperatures.
U.S. Pat. No. 5,096,943 describes a rubber composition obtained by suspending a metal salt of α,β-ethylenically unsaturated carboxylic acid in an aliphatic hydrocarbon solvent and mixing the suspension with a polymer solution. This method necessitates removal of the solvents at the end of liquid mixing.
U.S. Pat. No. 5,096,201 uses a rubber composition obtained by previously mixing and dispersing a metal salt of unsaturated carboxylic acid in liquid rubber, and kneading it in a rubber compound. Presumably the liquid rubber used herein can reduce the rebound of the golf ball obtained therefrom.